Animated Solution for Chemistry - s and p-Block Elements: Which one of the following is the correct statement ?

This question is a fantastic test of your foundational knowledge across different groups of the p-block and s-block elements. It requires us to evaluate four distinct chemical statements and identify the single correct one. Let's break down each option to understand the underlying chemistry.

The Nature of Boric Acid

The first statement claims that boric acid ($H_{3}B{O}_3$) is a protonic acid. A protonic (or Brønsted-Lowry) acid is a substance that donates its own hydrogen ions ($H^{+}$) in an aqueous solution, much like $HCl$ or $H_{2}S{O}_4$.

However, boric acid is a classic exception! The boron atom in $H_{3}B{O}_3$ is electron-deficient; it only has six electrons in its valence shell. Instead of releasing a proton, it acts as a Lewis acid by accepting a lone pair of electrons from a hydroxide ion ($O{H}^{-}$) of a water molecule.

Because it forces the water molecule to release the proton rather than donating its own, statement (a) is incorrect.

Beryllium's Coordination Limit

The second statement suggests that beryllium can exhibit a coordination number of six. To verify this, we must look at the electronic configuration of beryllium ($Z=4$).

Beryllium's configuration is $1s^{2}2s^2$. Its valence shell is the second principal quantum shell ($n=2$), which contains only one $2s$ orbital and three $2p$ orbitals. This gives a total of exactly four orbitals available for bonding.

Crucially, there are no $d$-orbitals in the second shell. Without $d$-orbitals, beryllium cannot expand its octet to accommodate more than four electron pairs. Therefore, its maximum coordination number is strictly restricted to four. Statement (b) is incorrect.

Bridged Chloride Structures

Statement (c) states that chlorides of both beryllium and aluminium have bridged structures in the solid phase. Let's investigate.

Both $BeC{l}_2$ and $AlC{l}_3$ are highly electron-deficient in their simple monomeric forms. Beryllium only has four valence electrons around it in $BeC{l}_2$, and aluminium has six in $AlC{l}_3$. Nature abhors a vacuum, and molecules abhor an incomplete octet!

To achieve stability, these molecules polymerize or dimerize in the solid state. They do this by forming coordinate (dative) bonds. A chlorine atom from one molecule uses its lone pair of electrons to form a bond with the empty orbital of the metal atom in an adjacent molecule.

This results in a polymeric chain $(BeC{l}_2{)}_n$ for beryllium and a dimer $A{l}_{2}C{l}_6$ for aluminium. Because both indeed form these bridged structures to satisfy their electron deficiency, statement (c) is absolutely correct!

The Real Inorganic Benzene

The final statement claims that $B_2{H}_6\cdot 2N{H}_3$ is known as 'inorganic benzene'.

This is a factual error. The title of 'inorganic benzene' belongs to Borazine, which has the chemical formula $B_3{N}_3{H}_6$. Borazine earns this nickname because its planar, hexagonal ring structure of alternating boron and nitrogen atoms is isoelectronic and isostructural with the carbon ring of benzene ($C_6{H}_6$).

The compound given in the option, $B_2{H}_6\cdot 2N{H}_3$, is actually an adduct formed between diborane and ammonia, which exists as an ionic salt formulated as $[H_{2}B(N{H}_3{)}_2{]}^{+}[B{H}_4{]}^{-}$. Thus, statement (d) is incorrect.

By systematically eliminating the false statements, we confidently arrive at option (c) as the correct answer.